The quality or state of being without limits or boundaries; endless extent or duration.
From 'extend' plus '-less' (meaning without) plus '-ness' (creating an abstract noun). This double suffix construction shows English's ability to layer meaning.
This word is beautifully paradoxical—'extend' means to stretch something out, but 'extendless' means something that has no limit to its extent, so it's kind of the ultimate extension!
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