Green’s Dictionary of Slang

hammock n.

[resemblance; but note the hammock is for a ‘lazy cunt’, punning on cunt n. (4)/cunt n. (1), which is ‘lazy’ because it is not available for intercourse]

(UK juv.) a sanitary towel; thus phr. the hammock is swinging, to be menstruating.

Show quotations
K. Elgin Twenty-eight Days 17: Some other [terms for menstruation] known are: ‘riding the cotton bicycle,’ ‘the hammock is swinging’ (from the shape of the sanitary pad).
OnLine Dict. of Playground Sl. 🌐 hammock n. sanitary towel (term usu. used by males) f. ‘hammock for a lazy cunt’.