An Etymological Dictionary of the German Language/dick
dick, adj., ‘thick, stout, corpulent,’ from MidHG. dic, dicke, adj., ‘thick, dense, frequent,’ OHG. dicchi, ‘thick, dense’; in Eng. too the double meaning of the adj. obtains; comp. OIc. þykkr, þjǫkkr, AS. þicce, E. thick. Corresponds to OIr. tiug (from *tigu), ‘thick,’ so that we must presuppose a Goth. *þiqus. Beside which the double sense, ‘thick, dense,’ makes the kinship with dicht probable. In OHG. the meaning ‘dense’ has been preserved in Dickicht, lit. ‘a place densely overgrown’ (orig. used by sportsmen); in MidHG. dicke is the equiv. term.