Unintentionally or without meaning to; doing something almost automatically or against one's better judgment (a commonly used adverb in spoken Japanese).
ついついテレビを見過ぎてしまった。
I ended up watching too much TV without meaning to.
スーパーで安いとついつい買ってしまう。
If it's cheap at the supermarket, I tend to buy it without thinking.
彼の冗談に反応してしまい、ついつい笑ってしまった。
I reacted to his joke and couldn't help laughing.
ダイエット中でもついつい間食をしてしまう。
Even when I'm dieting, I end up snacking without meaning to.