Both meanings derive from an Anglo-Saxon word. But in the Middle Ages, a sense also developed referring to a person who was "light" in character - specifically, in relation to sexual matters - and this is used several times by Shakespeare. So when Dromio of Syracuse describes a courtesan as a "light wench", he is not referring to her weight (The Comedy of Errors, IV.iii.52). Berowne, too, comments that "light wenches may prove plagues to men forsworn' (Love's Labour's Lost, IV.iii.361). When there is an accompanying word, such as "wenches" or "lust", we have a clue to the intended sense. But when the word occurs on its own, then we have to look carefully at the context if we are not to miss the nuance. "Women are light at midnight", says Lucio (Measure for Measure, V.i.278). And at one point in Henry IV Part 2 (II.iv.290), Falstaff greeting Prince Hal says: "by this light - flesh and corrupt blood, thou art welcome." He lays his hand upon Doll Tearsheet as he speaks. "By this light" isn't just an innocent oath.
David Crystal is author, iwth Ben Crystal, of Shakespeare's Words, published by Penguin