Rashi on Exodus 18:18/רש"י על שמות י״ח:י״ח

1נבל 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)‎.

2גם אתה THOU ALSO — The word גם “also” was added in order to include Aaron and Hur and the seventy elders (cf. Mekhilta).

3כי כבד ממך FOR [THIS THING] IS TOO HEAVY FOR THEE — its weight is far more than thy strength can bear.