The surname given to Charles, natural son of Pépin d'Héristal, for his victory over the Saracens, who had invaded France under Abd-el-Rahman in 732. It is said that Charles “knocked down the foe, and crushed them beneath his axe, as a martel or hammer crushes what it strikes.”
Judas Asmonæus for a similar reason was called Maccabæcus (the Hammerer). M. Collin de Plancy says that Charles, the palace mayor, was not called Martel because he martelé (hammered) the Saracens, but because his patron saint was Martellus (or Martin). (Bibliothéque des Légendes.)
Avoir se mettre martel en tête. To have a bee in one's bonnet, to be crotchety. Martel is a corruption of
Martin, an ass, a hobby-horse. M. Hilaire le Gai says, but gives no authority, “Cette expression nous vient des Italiens, car en Italien martello signifie proprement `jalousic.' ”
“Ils portent des martels, des capriches.” —Brantome: Des Dames Gallantes.
“Telle filles ... pourroient blen donner de bons martels à leurs pauvres marys.” —Brantome: Des Dames Gallantes.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Martel from Fact Monster: