Here’s a set of words that have perplexed some writers their entire lives: lifelong vs. life-long vs. life long.
Most grammarists agree that lifelong – meaning lasting through one’s existence, as in a lifelong friendship – should be one word rather than two words or hyphenated.
Confusion over the spelling arises because punctuation rules say that compound words, when used as an adjective, usually require a hyphen, as in well-known man. “Usually” is the operative word here, as lifelong is an exception. The same applies to other “long” words that show a length of time, such as daylong, monthlong, weeklong and yearlong.
Life long is never correct. Typically compound words with a space consist of two nouns, such as ice cream.
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