if'n · contraction1
From if + and (when).
if'n · conjunction2
Uncertain. The spelling ef'n is attested since at least 1909. Perhaps a compound of if and an (“if”) (a variant, attested since Middle English, of and (“if”); compare an if and gin (“if”)); if an is attested since at least the 1700s. Alternatively, perhaps a compound of if and -in', generalized from conjunctional uses of considering, excepting, and so on, or of if and -en as in offen (“off”), outen (“out”) (which goes back to Old English ūtan), etc. Compare less'n. Lastly, as with the etymology above, it may be a contraction of if (and) when.
Etymology from Wiktionary, CC BY-SA 4.0.