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. Finnish mallas, Old Church Slavonic mlato are considered to be borrowed from Germanic.
mid-15c., "to convert grain to malt," from malt (n.). Meaning + "to make with malt" is from c. 1600. Related: Malted; malting. Malt liquor (which is fermented, not brewed) first attested 1690s. Malted "a drink with malted milk" is from 1945.