Green’s Dictionary of Slang

1. in pl., the testicles.

1890–1904

Farmer & Henley Sl. and Its Analogues.

2. the vagina, thus sexual intercourse.

1849		Sam Sly 20 Jan. 2/3: He advises William Ha—d—e, a two-faced fellow, alias Plug- hole, [...] not to be so fond of running after the servant girl at No. 1, Duke- street, Adelphi, for the sake of the nick-nacks he can get out of her. It does not look the thing for him to take the young woman into the parks late at night;.
1890–1904		Farmer & Henley Sl. and Its Analogues.

3. (US) a male homosexual, esp. a promiscuous one.

1966

F. Elli Riot (1967) 177: Every nicknack in the joint must be over there.

4. (US prison) a man who is raped frequently in prison.

1972

B. Rodgers Queens’ Vernacular 161: Sexually oppressed, constantly raped victim; usually straight [...] nick-nack.

5. (US prison, also knick-knack) one who does not fit in with the group; an outsider.

2000		Other Side of the Wall: Prisoner’s Dict. July 🌐 Knick-Knack: Same as ‘lame.’.
2006		Prison Slang Mommyblogger mydogharriet.blogspot.com 2 Mar. 🌐 That knick-knack needs to get mud checked for getting on your leg and then cappin’ it with no explanation.