lurker

Concise Oxford English Dictionary © 2008 Oxford University Press:
lurk/lɜːk/

▶verb

• 1 be or remain hidden so as to wait in ambush.

• 2 be present in a latent or barely discernible state.

  ■ informal read the postings on an online forum without contributing any messages.

▶noun Austral./NZ informal a dodge or scheme.

– derivatives
lurker noun.

– origin ME: perh. from lour + the frequentative suffix -k (as in talk).