Sir Dafydd ap Llewelyn ap Hywel (c. 1380 – 25 October 1415), better known as Dafydd Gam or Davy Gam, was a Welsh nobleman, a prominent opponent of Owain Glyndŵr. He died at the Battle of Agincourt fighting for King Henry V, King of England in that victory against the French.
The name "Gam" is taken from a Welsh word for "lame/deformed" (from which "gammy", as in "gammy leg", may be derived). Regarded as a traitor ("Crooked David") by some Welshmen, he is regarded as a hero by others; his reputation has waxed and waned with those of his enemy Owain Glyndŵr and his ally King Henry V.
Dafydd Gam was a member of one of the most prominent Welsh families in Breconshire. His recent pedigree was 'Dafydd Gam ap Llywelyn ap Hywel Fychan ap Hywel ap Einion Sais', but beyond that the family claimed an ancient Welsh lineage going back to the Kings of Brycheiniog. Dafydd Gam was the grandson of Hywel Fychan, who held the manor of Parc Llettis near Llanover in Monmouthshire near Abergavenny, and fourth in descent from Einion Sais who held a castle at Pen Pont on the River Usk near Brecon and who had served at both the Battle of Crecy and the Battle of Poitiers. Their power base had developed mainly as consistently loyal supporters of the de Bohun family who were both earls of Hereford and Lords of Brecon from the 13th century onwards. Dafydd Gam's father, Llywelyn ap Hywel, purchased the estate of Penywaun near Brecon and Dafydd is thought to have been born there. His family was described as "a striking example of a native family that flourished under the rule of an English aristocratic family." Under Llywelyn ap Hywel, the family's traditional loyalty was transferred to the new Lord of Brecon, Henry Bolingbroke, who had married Mary de Bohun in the 1380s. Some say Dafydd was previously in service to Henry's father John of Gaunt and, having killed a rival in Brecon High Street, had to leave Wales temporarily.