To lose, or to take a loss. The L comes from the word lose/ loss.

Intelligence agency term for "psychological operation". A government or corporate-sponsored operation, usually taking the form of a "terrorist attack" or "crazed gunman on a spree", with the intent of panicking the public into demanding more police and laws inhibiting freedom. Psyops are usually carried out by drugging a civilian or group of civilians with aggression-promoting drugs, psyching them up, arming them, and sending them out to commit mayhem. Government-sponsored terrorism. See also blackshirts, conspiracy

Catching an L means catching a loss. In this sense, you catch an L if you say something stupid or, if you try to sound intelligent but are proved wrong.

L mean "loss" or "lose", depending on the context it is used in.

L is a letter.

To simply lose. When things Do not go as planned or when one has no choice but to decline an invitation for an appealing event.