1. [late 18C+] a reverberating blow.
2. [mid-19C+] (Aus.) a large piece, a share, a portion.
3. [1930s–40s] (US, also whangdanger) an excellent thing; also as adj.
4. [1930s+] (orig. US) the penis, usu. large [the aggressive image of the penis as that which ‘hits’ the vagina].
see step on one’s dick under dick n.1
see have a whack at under whack n.1