Entry 1 sense Webster, 1913 Mother-in-law /mŭˈ -thər-ĭn-lôˌ/ · Moth·er-in-law · IPA /ˈmʌ.ðəɹ.ɪnˌlɔː/ 01 n. The mother of one's husband or wife. 1. The mother of one's husband or wife.