The word 'forth' is one of the oldest and most fundamental directional adverbs in English, descending from Old English 'forþ' (forward, onward, out), from Proto-Germanic *furþa- (forward), from PIE *per- (forward, through). It means 'out and forward' — away from a starting point and into the open, onward into time or space.
Old English 'forþ' was far more common in everyday speech than its modern descendant. Anglo-Saxon texts use it abundantly: warriors go forth to battle, kings send forth commands, ships sail forth over the sea. In modern English, 'forth' has retreated into fixed phrases and formal registers — 'set forth,' 'put forth,' 'bring forth,' 'call forth,' 'come forth,' 'henceforth,' 'thenceforth,' 'and so forth,' 'back and forth,' 'hold forth.' In unrestricted modern speech, 'forward' or 'out' typically replace it.
The PIE root *per- is one of the most productive in the language family, generating words for forward motion, passage, and transition across every daughter language. In the Germanic branch: 'forth,' 'further' (comparative: more forward), 'first' (superlative: most forward), 'for' (originally spatial: in front of), 'fore' (in front), 'before' (by the front), 'far,' 'from,' and 'forward.' In Latin: 'pro-' (forward, for), 'per-' (through), 'prīmus' (first), 'prior' (earlier, more forward in time), and 'prae-' (before). In Greek: 'pro-' (before), 'perí' (around), 'pára' (beside, past). In Sanskrit: 'pra-' (forward), 'para-' (beyond).
The grammatical relationship between 'forth,' 'further,' and 'first' is significant and largely forgotten. 'Forth' is the positive degree (forward), 'further' is the comparative (more forward), and 'first' is the superlative (most forward). This makes 'first' etymologically 'the most forth' — the one who has gone forward the most, the one at the front. The modern English speaker who uses 'first,' 'further,' and 'forth' in the same paragraph is unconsciously employing three degrees of a single ancient concept.
Compounds with 'forth' preserve distinct medieval senses. 'Forthcoming' (about to come forth — approaching, available) dates from the fifteenth century. 'Forthright' (going straight forth — direct, frank) from Old English. 'Forthwith' (with the going forth — immediately, without delay) from Middle English. 'Henceforth' (from this point forth — from now on) and 'thenceforth' (from that point forth) from Middle English.
The phrase 'to set forth' has two important meanings: to begin a journey (to set oneself forward) and to state or describe something in detail (to put ideas forward for examination). 'To bring forth' means to produce or give birth — to bring something out into the world. 'To put forth' means to extend or propose — to push something forward for consideration. Each phrase animates 'forth' with a different verb, creating distinct but related meanings from the same directional concept.
The word's retreat from everyday speech into fixed phrases is typical of archaic adverbs in English. Like 'hence,' 'thence,' 'whence,' 'hither,' 'thither,' and 'whither,' 'forth' belongs to an older system of directional terms that has been largely replaced by prepositional phrases ('from here,' 'to there,' 'forward'). But its survival in idioms ensures that the word remains familiar, if no longer freely productive.