used before adjectives and adverbs to say that something is more than is good, necessary, possible, etc.

When I've finished painting the bathroom, I'm going to do the kitchen too.

I'm not too sure if this is right.

I'm just going out—I won't be too long.

Mary will be only too glad to help.

She's none too clever.

potential customers who will not be too pleased about your lack of attention.

It's unlikely anyone's going to be too concerned about a little thing like that.

used to comment on something that makes a situation worse

She broke her leg last week—and on her birthday too!

used to emphasize something, especially your anger, surprise or agreement with something

‘He did apologize eventually.’ ‘I should think so too!’

‘She gave me the money.’ ‘About time too!’

LEARNit Dictionary