What does "better half" mean?

Also, better part. The larger amount or majority of something, as in I won't be long; the better half of this job is complete, or I have spent the better part of my life in this city. Sir Philip Sidney used the first term in Arcadia (1580): “I ... shall think the better half of it already achieved.” The variant appears in a well-known proverb, discretion is the better part of valor.

Also, my better half. One's (my) spouse, as in I'm not sure if we can go; I'll have to check with my better half. Originally this expression meant “a close friend or lover,” and by the 16th century it referred to either a wife or lover. Sidney used it in this way, again in Arcadia: “My dear, my better half (said he), I find I must now leave thee.” Today it tends to be used lightly for either husband or wife. “Late 1500s”