-in-lawa suffix meaning 'related by marriage', as grandparents-in-law.
1894 Blackw. Mag. Jan. 24The position of the ‘in-laws’ (a happy phrase which is attributed..to her Majesty, than whom no one can be better acquainted with the article) is often not very apt to promote happiness. 
1898 Daily News 7 Jan. 4/7 ‘Don't live with them’—with the ‘in-laws’. 
1898 Westm. Gaz. 1 Apr. 1/3Everyone living is either an ‘in-law’ himself, and therefore bound to possess corresponding ‘in-laws’, or his ‘in-law’ potentiality remains intact. 
Ibid. ,‘In-lawry’ is the common fate of the entire human race. 
1912 D. Canfield Squirrel Cage I. i. 10Her mother felt the usual in-law conclusion about her daughter's life. 
1926 C. Sidgwick Sack & Sugar i. 9Eva had invited her future in-laws, male and female, to five o'clock a day or two before her wedding. 
Ibid. vii. 77He is lost to everyone but his wife and his in-laws. 
1939 N. S. Colby Remembering i. 19Robert's sister Catherine has to mix in-law amenities with adoring love. 
1952 A. Grimble Pattern of Islands 205A retired..policeman, married to a local woman and on a visit to his in-laws. 
1954 M. Gluckman in E. E. Evans-Pritchard Inst. Primitive Society vi. 69This immediately established links of in⁓lawship with people who were, by standards of blood-ties, his enemies. 
1957 V. W. Turner Schism & Continuity inAfr. Soc. ix. 258Links of in-lawship between villages. 
1964 Gould & Kolb Dict. SocialSci. 368/1An indefinite number of affinal relatives or ‘in-laws’. 
1965 G. Melly Owning-Up xv. 190He was living with his wife and baby daughter in his in-laws' semi out at Mill Hill. 
1970 Daily Tel. 8 May 17How difficult and unnatural are in-law relationships! 
1970 G. F. Newman Sir, You Bastard iii. 91His in-laws bought the furniture for the new house. 
1972 R. Milner in W. King Black Short Story Anthol. 374The small salary added to his wife's keeping his in-law landlords quiet as he sweated through his first year of accounting. 
ORIGIN: from in  preposition law  noun Anglo-Norman  en ley , Old French  en loi  (de mariage ) in law (of marriage).
-in-law
Adjective
(not comparable)
- postpositive Related through marriage. - cousin-in-law