pong
Concise Oxford English Dictionary © 2008 Oxford University Press:
pong /pɒŋ/ Brit. informal
▶noun a strong unpleasant smell. ▶verb smell strongly and unpleasantly.
– derivatives
pongy adjective (pongier, pongiest).
pongy adjective (pongier, pongiest).
– origin early 20th cent.: of unknown origin.
'pong' also found in these Oxford entries:

