boner (n.) "blunder," 1912, baseball slang, probably from bonehead.
Meaning "erect penis" is 1950s, from earlier bone-on (1940s), probably a
variation (with connecting notion of "hardness") of hard-on (1893).
http://www.etymonline.com/index.php?term=boner
I had an old 6th grade teacher in the early 70s that still used the word
boner in the sense of as a blunder. She would say someone made a boner,
once in a while. I've personally never heard anyone else use it that
way. I've heard bonehead once in a while.
And I don't understand where "hard-on" comes from. Sounds odd, the
"-on" part. Always has sounded strange to me. Why would someone have
said that, and it caught on? I didn't find anything from Google about
it's origin other than above it says it originated in 1893.... Also,
some people say someone was hard on someone, meaning they gave them a
difficult time.