古英語のsceacga「髪、毛むくじゃら」から、原ゲルマン語*skaggija「口ひげ、ひげ」から、語源的にはPIE*skek「揺らす、振る」から。
And te boot, amydde te water, was shaggid. [Wyclif]Compare shake it in U.S. blues slang from 1920s, ostensibly with reference to dancing. But compare shag (v.), used from 1610s in a sense "to roughen or make shaggy." Also the name of a dance popular in U.S. 1930s and '40s. Related: Shagged; shagging.