Meaning/Usage: To tell someone to wait; to slow down; an expression used before providing a different point of view or opinion
Explanation: This is fairly straightforward. "Sec" in this context is one "second." You're basically telling someone to wait because you are about to say something else.
Hold on a sec, I thought you were staying home tonight?
Hold on a sec, are you serious?
Hold on a sec. This is different from what you told me yesterday.
A.
Chip is going to draft a quarterback in the first round?
B.
Hold on a sec. I swore he was going to draft a running back first.
A.
It's a surprise for me too, but I'm pretty sure about it.