"grain (usually barley) in which, by heat, the starch is converted to sugar," Old English malt (Anglian), mealt (West Saxon), from Proto-Germanic *maltam (source also of Old Norse malt, Old Saxon malt, Middle Dutch, Dutch mout, Old High German malz, German Malz "malt"), possibly from PIE root *mel- (1) "soft" via the notion of "softening" the grain by steeping it in water before brewing.
By the addition of hops, and the subsequent processes of cooling, fermentation, and clarification, the wort is converted into porter, ale, or beer. The alcoholic fermentation of the wort without the addition of hops and distillation yield crude whisky. [Century Dictionary]
Finnish mallas, Old Church Slavonic mlato are considered to be borrowed from Germanic. Meaning "liquor produced by malt" is from 1718. As an adjective, "pertaining to, containing, or made with malt," 1707; malt liquor (which is fermented, not brewed) is attested from 1690s.
mid-15c., malten, "to convert grain to malt," from malt (n.). Meaning "to make with malt" is from c. 1600. Related: Malted; malting. Malted (n.) "a drink with malted milk" is by 1945.