surreptitious
Concise Oxford English Dictionary © 2008 Oxford University Press:
surreptitious /ˌsʌrəpˈtɪʃəs/
▶adjective covert or clandestine.
– derivatives
surreptitiously adverb,
surreptitiousness noun.
surreptitiously adverb,
surreptitiousness noun.
– origin ME (in the sense ‘obtained by suppression of the truth’): from L. surreptitius (from surripere, from sub- ‘secretly’ + rapere ‘seize’) + -ous.
'surreptitious' also found in these Oxford entries:

