Forth means onward and out, and it belongs to older or formal English. Going out from where you are.
It survives mainly in set phrases: back and forth, and so forth, hold forth, and set forth on a journey. Almost nobody uses it alone any more. Bring forth and put forth are biblical in flavour. Take care with the spelling, since fourth is the ordinal number and the two sound identical, which is one of the commonest homophone errors in English.
1adverbforthOut, or forward from a place, in formal or older use.
“They set forth at first light.”
Proto-Indo-European
*pŕ̥-to-
Proto-Germanic
*furþą
Proto-West Germanic
*forþ
Old English
forþ
Middle English
forth
English, today
forth
From Middle English forth, from Old English forþ, from Proto-West Germanic forþ, from Proto-Germanic furþą, from Proto-Indo-European pŕ̥-to-, from per-. Cognates include Dutch voort and German fort. See also ford.
Etymology from Wiktionary, CC BY-SA