a 16c. merger of two Middle English words, with the form of infamous "not well-known" (early 15c.) and the sense of infamis (late 14c.), "of ill repute, famous for badness." Infamous is from Medieval Latin infamosus, from in- "not, opposite of" (see in- (1)) + Latin famosus "celebrated" (see famous). Infamis is from Latin infamis "of ill fame" (see infamy).
Meaning "causing infamy" is from 1550s. As a legal term, "disqualified from certain rights of citizens because of conviction for certain crimes" (late 14c.). The neutral fameless (in the sense original to infamous) is recorded from 1590s. Related: Infamously.