Left: mouth 口, right: prosperous 亨 (Who makes big/high 高 things to accomplish 了 is prosperous.) [了 = The still arm-less 了 child 子 should be completed.]
Mnemonic
Out of the mouth 口 below the nose 亠, this mouth 口, comes out 了 a groan.
Perhaps to be understood as a combination of tall/big 高 and finish 了. (了 = The still armless 了 child 子 should be finished.) So if big things are finished, you are prosperous/successful.
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