If something that you want to say is on the tip of your tongue, you think you know it and that you will be able to remember it very soon.

(想说的话)就在嘴边(很快就会记起来)

Her name is on the tip of my tongue.

她的名字就在我的嘴边却一时说不出来。

It was on the tip of Matt's tongue to say the boss was out.

马特想说老板出去了，话到嘴边却没说出口。