חֲשַׁשׁ

Isaiah 5:24 וַחֲשַׁ֤שׁ and dry grass

Therefore, as a tongue of fire consumes the straw, and as dry grass shrivels in the flame, so their roots will decay and their blossoms will blow away like dust; for they have rejected the instruction of the LORD of Hosts and despised the word of the Holy One of Israel.

Here: A rare word, found only here and Isa 33:11. It means dried grass or hay, a light material that collapses as it burns. KJV's 'chaff' is a guess from the context.

Isaiah 33:11 חֲשַׁ֖שׁ dry grass

You conceive chaff; you give birth to stubble. Your breath is a fire that will consume you.