The very tip or end of something (colloquial). Common in everyday speech (e.g., 'the tip of a pen').
ペンの先っちょを少し折ってしまった。
I accidentally broke off the very tip of the pen a little.
かばんの先っちょに鍵が引っかかっているよ。
A key is caught on the edge (tip) of your bag.
この道は先っちょまで行って引き返そう。
Let's go to the very end of this road and then turn back.
子どもはお菓子の先っちょばかり食べてしまった。
The child ate only the very end/piece of the snack.