Rashi on Yisro 18:18

English: נבל תבל — Explain this as the Targum does: thou will certainly become weary. Its meaning expresses the idea of withering, old French flestre, just as. (Jeremiah 8:13) “and the leaf is withered (נבל)”; (Isaiah 34:4) “as withereth (כנבל) the leaf from off the vine” — the meaning being that it becomes shrivelled through the heat and through the frost, and so its strength diminishes and it becomes, as it were, weary, and falls from off the vine (מגפן)‎. גם אתה THOU ALSO — The word גם “also” was added in order to include Aharon and Hur and the seventy elders (cf. Mekhilta). כי כבד ממך FOR [THIS THING] IS TOO HEAVY FOR THEE — its weight is far more than thy strength can bear.