The Collaborative International Dictionary of English v.0.48:

Laughing \Laugh"ing\, a. & n.
   from Laugh, v. i.
   [1913 Webster]

   Laughing falcon (Zool.), a South American hawk
      (Herpetotheres cachinnans); -- so called from its notes,
      which resemble a shrill laugh.

   Laughing gas (Chem.), nitrous oxide, also called
      hyponitrous oxide, or protoxide of nitrogen; -- so called
      from the exhilaration and laughing which it sometimes
      produces when inhaled. It has been much used as an
      anaesthetic agent, though now its use is primarily in
      dentistry

   Laughing goose (Zool.), the European white-fronted goose.


   Laughing gull. (Zool.)
   (a) A common European gull (Xema ridibundus); -- called
       also pewit, black cap, red-legged gull, and sea
       crow.
   (b) An American gull (Larus atricilla). In summer the head
       is nearly black, the back slate color, and the five outer
       primaries black.

   Laughing hyena (Zool.), the spotted hyena. See Hyena.

   Laughing jackass (Zool.), the great brown kingfisher
      (Dacelo gigas), of Australia; -- called also giant
      kingfisher, and gogobera.

   Laughing owl (Zool.), a peculiar owl (Sceloglaux
      albifacies) of New Zealand, said to be on the verge of
      extinction. The name alludes to its notes.
      [1913 Webster]

WordNet (r) 3.0 (2006):

laughing gull
    n 1: small black-headed European gull [syn: laughing gull,
         blackcap, pewit, pewit gull, Larus ridibundus]