Entry from OED Online

her, pers. pron., 3rd sing. fem., dat.-acc.

SECOND EDITION 1989

[4.] b. her indoors (also 'er indoors) Brit. colloq., one's wife or girlfriend; in extended use, applied to any woman occupying a position of authority who is regarded as domineering.
The phrase was popularized by the Thames Television series Minder (1979-93), in which the leading character Arthur Daley habitually referred to his wife as ‘her indoors’. The series' original writer, Leon Griffiths, app. first heard it used by ‘a taxi-driver drinking companion of his’ (Independent (1992) 16 June 13/6).

1979 L. GRIFFITHS Smaller they Are in Minder (television script, second draft) 10 May 2 That's what her indoors doesn't understand Terry. A young bird keep [sic] you feeling young. 1984 Guardian 17 Oct. 12/2 These days, her indoors (and Mr Walker too) are said to be seldom off the phone with words of wisdom for Mr MacGregor. 1986 R. SPROAT Stunning Punters 156, I was taking Her Indoors out for a day at the Zoo. 1988 Times 2 Nov. 21/1 She [sc. the mistress of a Great Artist] is immortalized in oils, while 'Er Indoors cleans up the mess afterwards. 1992 Pilot July 36/1, I began to consider buying a single-seat kitplane, a microlight, or even a taildragger. But there were howls of disagreement from Her Indoors and the rest of the tribe.