One way to hold sustenance: a hand under something that would otherwise sink. Not a gift, not a treat — the steady supply that keeps the thing from going down. Take the hand away and it stops. A body stops without food; a club stops without members and money. Sustenance names whatever is doing the holding up.
From the word’s history
Wiktionary traces it to Old French sustenance, from sustenir, from Latin sustinere — equivalent to sustain + -ance. Sustain is literally to hold up from below, which is exactly the image both senses use.
來源把它追到古法文 sustenance，再到拉丁文 sustinere，並說它等於 sustain 加 -ance。sustain 的字面就是「從下面撐住」，這正好是兩個意思共用的畫面。
Wiktionary entry for sustenance: From Middle English sustenaunce, from Old French sustenance, from sustenir with the suffix -ance, from Latin sustinere; equivalent to sustain + -ance — as supplied in the evidence packet
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