Green’s Dictionary of Slang

1. a postman, a courier.

1864, 1867, 1870		Hotten Sl. Dict.
1873		Sl. Dict. 335: Walker a letter-carrier or postman. From an old song, called, ‘Walker, the twopenny postman.’.
1895		Crowe Aus. Sl. Dict. 92: Walker, a postman.

2. a man, often rich, invariably personable and socially acceptable, who accompanies the wives of prominent men to parties, on shopping expeditions, to the theatre etc.

1978		A. Maupin Tales of the City (1984) 128: He was renowned in the social columns as an escort of widows [...] ‘Well,’ purred Peter, ‘haven’t the walkers crawled out of the woodwork tonight.’.
1989		T. Blacker Fixx 312: Yours truly accompanies the old wife abroad [...] A walker, she calls me.
1996		P. Temple Bad Debts (2012) [ebook] ‘I’ve met Gorman [...] He’s a walker for high-society hags’ ‘A walker?’ ‘Takes them to the theatre, to parties. When their husbands are too busy fucking the secretary’.