lurker

noun

1. One who lurks.

   “It troubled me that there should have been a lurker on the stairs, on that night of all nights in the year, and I asked the watchman, on the chance of eliciting some hopeful explanation as I handed him a dram at the door, whether he had admitted at his gate any gentleman who had perceptibly been dining out?”

2. (Internet)A user who observes a community rather than participating; someone who reads or takes advantage of content on a website, newsgroup, etc. but does not contribute.

   “However, less than half of lurkers went online that often. Those who post also are more likely than lurkers to use the Internet for both work and pleasure. Posters use the Internet to communicate more with others than lurkers do.”

   “It’s not unusual for people to join video calls with their camera off and microphone muted, but such lurkers can at least be called upon to speak.”

3. (UK, archaic, dialectal)A small fishing-boat.
4. (UK, obsolete, slang)An impostor; a quack.

   “In every large town sham official documents, with crests, seals, and signatures, can be got for half-a-crown. Armed with these, the patterer becomes a ‘lurker,’—that is, an impostor; his papers certify any and every ‘ill that flesh is heir to.’”