1A (or one's) wife. Chiefly as an affectionate diminutive.
2A mistress; especially any one of a number of mistresses with whom a man has a long-lasting sexual relationship.
Especially with reference to Alexander Thynn (b. 1932), seventh Marquess of Bath: see note in etymology.
Mid 19th century; earliest use found in Putnam's Monthly Magazine. From wife + -let.