Short and thick, or blunt and thick at the edges rather than pointed or long.
From 'stub,' meaning a short thick piece left after something is cut or worn down. The -y suffix turns the noun into an adjective describing something with stub-like qualities.
Your pencil gets 'stubby' the more you use it—the word literally describes the end stage of wear, which is why painters love stubby brushes: they're worn to maximum usefulness.
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