Green’s Dictionary of Slang

header n.

1. [early 19C] a blow to the head.

2. going lit. or fig. ‘head-first’.

(a) [mid–late 19C] in fig. use of sense 2b, a bet, usu. spontaneous and unthought-out.

(b) [mid-19C+] a head-first dive, usu. into water.

(c) [1920s] a social decline.

(d) [1990s+] one who jumps, usu. for the purposes of suicide, off a high building, bridge etc.

3. [1950s+] (US) an act of oral copulation [head n. (2c)].

4. [1960s+] (Irish) a psychotic, an unstable or bizarre person.