The mother-in-law language is an unusual feature of the Australian aboriginal language Dyirbal (as well as a few related languages). It is a taboo avoidance language - with certain relatives, namely cross-gender parents-in-law and cousins, if they are within hearing distance the mother-in-law language (known as DyalNuy) must be used. It has the same grammar but the lexicon is entirely different and much smaller; one word in DyalNuy corresponds to several words in normal Dyirbal. The intent is to symbolically conceal speech because it is taboo to speak to these relatives.

The mother-in-law language has not been in heavy use since the 1930s and this is considered a sign of the decline of the Dyirbal language and culture.