Origins
The English adjective 'long' descends from Old English 'long' (also 'lang'), from Proto-Germanic *langaz, which traces to the Proto-Indo-European form *dlongʰos, meaning simply 'long.' The PIE form is notable for its initial consonant cluster *dl-, which was simplified differently in the various daughter languages: Germanic dropped the *d- to produce *langaz, while Italic also dropped it (through a different phonological process) to produce Latin 'longus.' The Slavic branch, by contrast, preserved a version of the cluster: Old Church Slavonic 'dlŭgŭ' (long) retains the *d-, as does its modern descendant Russian 'длинный' (dlinnyy, long).
The fact that English 'long' and Latin 'longus' both derive from the same PIE root means that English has effectively inherited this word twice. The native Germanic 'long' arrived through unbroken oral transmission from Proto-Germanic, while the Latin form entered English through French and scholarly borrowings: 'longitude' (long-measurement), 'elongate' (to make long), 'oblong' (somewhat long), 'longevity' (long-livedness), and 'prolong' (to extend in length). This double inheritance is common in English — 'whole' and 'hale' from Germanic alongside 'holistic' from Greek, for instance — but 'long/longitude' is one of the clearest examples.
The Proto-Germanic cognates are nearly identical across the family: German 'lang,' Dutch 'lang,' Swedish 'lång,' Danish and Norwegian 'lang,' Icelandic 'langur,' and Gothic 'laggs.' The uniformity is striking — this is a word that has resisted change across three millennia of separate development in the Germanic languages.
Germanic Development
The noun 'length' is derived from 'long' with the abstract suffix '-th' (from Proto-Germanic *-iþō), the same suffix that creates 'width' from 'wide,' 'depth' from 'deep,' and 'strength' from 'strong.' The vowel change from 'long' to 'length' (o to e) reflects i-mutation, a prehistoric Germanic sound change in which a back vowel was fronted under the influence of an 'i' or 'j' in the following syllable.
The verb 'to long' (to yearn, to desire intensely) appears to be the same word but has a more complex history. Old English 'langian' meant 'to grow long, to extend,' and also 'to long for, to yearn,' the idea being that time grows long when one is waiting or desiring. The connection between physical length and emotional yearning — time stretching out painfully — is one of the most evocative metaphors embedded in English vocabulary. 'Longing' preserves this sense vividly.
The word 'belong' also contains 'long,' though the connection is not immediately obvious. 'Belong' comes from Middle English 'belongen,' from 'be-' (an intensifying prefix) plus 'longen' (to be fitting, to pertain to), from Old English 'gelang' (dependent on, belonging to). The semantic thread is the idea of 'going along with' or 'extending toward' — what belongs to you is what extends in your direction, what pertains to your domain.
Old English Period
In Old English, 'long' could describe both spatial extent and temporal duration, just as in Modern English. This dual application — 'a long road' and 'a long time' — is shared across all the Germanic languages and appears to go back to PIE. The metaphorical mapping of spatial length onto temporal duration is one of the most fundamental conceptual metaphors in human cognition, attested across unrelated language families worldwide.
The comparative 'longer' and superlative 'longest' are regular formations. Old English also had the adverb 'longe' (for a long time), which survived into Modern English as the adverb 'long' in expressions like 'long ago,' 'how long,' and 'so long' (as a farewell, possibly a calque from German 'so lange' or Arabic 'salaam'). The phrase 'so long' as a goodbye is first attested in American English in the mid-nineteenth century, and its exact origin remains debated.