Definition of roundheads:
(n) :
(informal, historical) A supporter of parliament during the English Civil War (in the 17th century).
(n) :
(slang) A male whose penis is circumcised.
(adj) :
(of a screw) Having a dome-shaped head.
(n) :
A roundhead screw, one with a domed head.
(n) :
A circular platform at the end of a pier or breakwater.
(n) :
Any of various agarics of the genus Stropharia.