From Latin 'clam' (secretly), from PIE *kel- (to conceal) — same root as 'cell,' 'cellar,' 'helmet,' and 'hell.'
Kept secret or done secretively, especially because illicit.
From French 'clandestin,' from Latin 'clandestīnus' (secret, concealed, done in hiding), from 'clam' (secretly, in private, apart from observation), from PIE *ḱel- (to cover, to conceal, to hide — the root of enclosure and hiding). The same PIE *ḱel- produced Greek 'kalyptein' (to conceal — as in 'apocalypse,' the un-concealing), 'calypso' (the concealing nymph of the Odyssey), Latin 'cēlāre' (to hide, to conceal — giving 'conceal' itself), 'color' (covering of a surface), Old English 'helan' (to cover — giving 'hell,' the covered place, the hidden underworld), German 'hehlen' (to conceal stolen goods), and 'helmet' (a small covering for the head). The suffix '-destīnus' may be related to 'destināre' (to make
The PIE root *ḱel- (to hide) gave English both 'clandestine' (through Latin 'clam') and 'hell' (through Germanic, as the hidden underworld). It also gave us 'cell,' 'cellar,' 'conceal,' and 'helmet' — covering and concealment in all their forms, from underground rooms to afterlife destinations.