Speakers of English treat longer as the ordinary comparative of long: a road that stretches farther, a wait that lasts more time, a list with extra items. The same spelling works as an adverb. In a handful of fixed phrases it has almost become a marker of time rather than a free adjective. No longer and any longer do not invite a picture of extra inches; they mark that a state has ended or that duration is still in play.
That everyday job is old. The interesting history sits in the form itself. English did not always attach the comparative ending to the unchanged stem long. For centuries the comparative had a different vowel.
Native stock, not a Romance loan
The word is inherited, not borrowed from French long or from Latin longus, even though those look close and mean much the same thing. The English adjective long comes from Old English lang or long, itself from Proto-Germanic *langaz. Latin longus is a cousin, not a parent. Both families are usually traced to a Proto-Indo-European base reconstructed near *dlonghos- or a related shape meaning extended in space or time, the same general stock as Sanskrit dīrgha and Greek dolichos. Specialists do not agree on the exact morphological link between the Germanic and Latin set and the Sanskrit and Greek set. The family resemblance is clear; the shared parts are not.
Old English already used the adjective for space and for time, and also, in some contexts, for height in people. That tall sense later faded and survives only at the edges of dialect, if at all. A regular Old English sound change turned many short a vowels into o before n, which is why Modern English writes long rather than lang. The same shift shows up in other pairs such as band and bond.
The comparative arrived with the adjective. Proto-Germanic had *langizô, the comparative of *langaz. Old English inherited that as lengra. The adverb had a related short form leng. None of this is a late invention. What is later is the spelling we now use.
The ending that used to change the stem
The -er on comparatives is not the same historical suffix as the agent ending on baker or singer. The comparative comes from Old English -ra in the masculine and -re in the feminine and neuter, from Proto-Germanic *-izon, ultimately from a Proto-Indo-European comparative adjective suffix. In early Germanic that suffix carried an i-element that triggered i-mutation: the stem vowel shifted toward the front of the mouth. A small set of Old English adjectives still showed the change regularly. Beside lang or long stood lengra and the superlative lengest. The same pattern produced ieldra from eald (old) and strengra from strang (strong).
By historical Old English times, umlaut in comparatives was already shrinking. Most adjectives simply added the suffix without touching the vowel. In Middle English the inherited mutated forms still competed with new analogical ones. Records of that period keep lenger and lengere as the expected descendants of lengra, used of extra length in space, of extra duration, and even of taller people. An analogical unmutated comparative longer, sometimes spelled langer, is found from at least the late 1300s. The analogical superlative longest is attested from the 1400s. Over the following generations the rebuilt pair longer and longest crowded out lenger and lengest. A bare adverb leng also died in early Middle English, replaced by forms aligned with the adjective.
This is regularization, not a new meaning. Speakers kept the positive form long in mind and attached the ordinary comparative ending to it, the way they did with most other short adjectives. Elder still shows the old vowel change; longer does not. Better is another survivor of a different stem plus the same old comparative machinery. Longer joined the regular majority.
Stable comparison, shifting grammar
The core comparison has been stable: more extent, more duration. Middle English lenger could still mean taller, following the older use of long for height. That human-height sense is not how most speakers use longer today. The productive growth is in time phrases. No longer is recorded from around 1300 with the sense not as before. Any longer belongs to the same cluster. In those constructions the comparative has grammaticalized: it no longer invites a measuring stick, even though the measuring-stick sense remains fully alive in a longer rope or a longer chapter.
A separate English noun longer, meaning a person who yearns, is built on the verb long plus the agent suffix -er, from Old English -ere. That verb, Old English langian, is probably connected with the adjective, via a Germanic verb meaning roughly to grow long or to stretch in feeling. The two words can look identical on the page. In speech the comparative usually keeps a stop after the nasal; the agent noun typically does not. They are homographs with different histories of the ending, not two senses of one lexeme.
French longer, a verb meaning to run along the side of something, is a different word again, a Romance formation on Latin longus. It is a reminder that lookalikes in neighboring languages need not be loans into English.
A cognate that kept the old vowel
German länger still fronts the vowel, as do Swedish längre, Danish længere, and Icelandic lengri. Dutch langer and West Frisian langer look closer to the modern English spelling. Those continental forms show that Proto-Germanic *langizô left a family of comparatives across the North Sea. English did not lose the comparative; it lost the umlaut that once marked it. The origin of the word longer is therefore less a tale of borrowing than of leveling: a native comparative rebuilt on the positive stem after the mutated Old English form had done centuries of work.
The useful takeaway is the date of that rebuild. From the late fourteenth century, writers could choose an analogical longer beside the inherited lenger. The analogical form won. When you write longer now, you are using the regularized Middle English outcome, not the umlauted Old English one that German still wears on its sleeve.