"blunder," 1912, baseball slang, probably from bonehead. Meaning "erect penis" is 1950s, from earlier bone-on (1940s), probably a variation (with connection notion of "hardness") of hard-on (1893).
n. a silly error; a gaffe. (See also pull a boner.) : What a boner! You must be embarrassed.
n. and bone. an erection. : He always gets a boner when he doesn't need it and never when he does.
Dictionary of American Slang and Colloquial Expressions by Richard A. Spears.Fourth Edition. Copyright 2007. Published by McGraw Hill.
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