1.  The European house martin. [1913 Webster]

2.  A bird without beak or feet; -- generally assumed to represent a martin. As a mark of cadency it denotes the fourth son. [1913 Webster]

martlet, n.
1 Heraldry an imaginary footless bird borne as a charge.
2 archaic a a swift. b a house-martin.

Etymology:
F martelet alt. f. martinet dimin. f. MARTIN