Green’s Dictionary of Slang

1. a drink composed of equal proportions of old (stout) and bitter.

1869		Man about Town 16 Oct. 44/1: We have often heard ‘drinks’ asked for at restaurants by funny names. Thus ‘Stout and Porter’ is always called for as ‘Cooper,’ and ‘Old and Bitter’ as ‘Mother-in-Law’.
1883		Bristol Magpie 3 May 4/2: [T]he fast young men of Bradford have just invented a new slang phrase. When calling for a ‘glass of old and bitter'’they now ask for ‘mother-in-law’.
1898		Belfast News-Letter 11 Apr. 6/5: The combination of old and bitter ale is ‘mother-in-law’.
1902		Dundee Courier 14 June 7/2: ‘Mother-in-law’ means old and bitter ales.
1933		‘William Juniper’ True Drunkard’s Delight 229: Mother-in-law is a mixture of old- and bitter-ales.
1955		Picture Post (ad for The Brewers’ Society) 23 July 22: Do you like a bitter? Or a stout? Or a mixture of both – a ‘mother-in-law’?

2. (S.Afr., Ind., also mother-in-law exterminator, mother-in-law’s hell-fire, mother-in-law masala) proprietary names for the hottest forms of chilli-based hot sauces or curry powders (masalas).

1987		Branford & Branford Dict. S. Afr. Eng. (3rd edn) 231/1: mother-in-law [...] Extra hot curry powder.
1988		Fair Lady 22 June (Supplement) 13: Try the [...] hottest curry powders this side of the Ganges: Mother in Law’s Hell Fire and Heaven’s Delight [DSAE].
1990		in Personality 18 June 27: The Valies are sniffing Mother-in-law Exterminator curry powder with a sort of desperate hope [DSAE].
1992		R. Mesthrie Lex. of S. Afr. Indian Eng. 30: Mother-in-Law masala, [...] A particularly pungent masala sold in Natal. Now rare. (A brand name, presumably created in jest, implying a sharp, bitter variety) [DSAE].
2006		P. Scholtz Demon of the Curry Powder 128: I will call it Mother-in-Law Curry Powder, a name to raise a sweat even on the most accustomed brow.
2007		P. de Bruyn Frommer’s S. Afr. 49: Indian curries, whose burn potential is indicated by such ingenious names as Honeymooners’ Delight (hot) and Mother-in-Law Exterminator (damn hot!).