Appearing most frequently at the beginning of sentences,
つい is a 副詞（ふくし） (adverb) that may be translated as ‘accidentally’, ‘unconsciously’, ‘absentmindedly’, or ‘against one’s better judgment’.
つい will often be paired with statements that use
てしまう, to emphasize that something was unintentional.
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ごめんタケシ君（くん）のクッキーが目（め）の前（まえ）にあったからつい食（た）べてしまった。
Sorry Takeshi-kun, your cookie was right in front of me, so I absentmindedly ate it.
先生（せんせい）に怒（おこ）られると、つい笑（わら）っちゃうんだよね。
When I am being scolded by the teacher, I accidentally laugh.
運転中（うんてんちゅう）に携帯（けいたい）を使（つか）ったらだめなのは分（わ）かっているんだけど、メールが来（く）るとつい携帯（けいたい）を見（み）ちゃう。
I know that I shouldn't use my phone while driving, but when I get a text I unconsciously look at my phone.
Although ‘accidentally’ is the most common meaning of
つい, it actually may be used to convey three different nuances. These are as follows:
はからず - Something that is uncalculated.
思（おも）わず - Something that is not thought about.
距離（きょり）
がわずかなさま - A small gap/interval.
In each of these meanings,
つい keeps a sense of time being the most important factor. Due to this,
つい will give the impression that whatever resulted in the ‘accidental’ action, will be something that happened momentarily. This is very close to the meaning of ‘lapse’ in English. ‘a lapse in time’, ‘a lapse in judgment’, ‘a lapse in forethought’.
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早（はや）く起（お）きて準備（じゅんび）をしても、つい遅（おく）れてしまう。
Even if I wake up early to get ready, I’ll be late due to a lapse of time.
電車（でんしゃ）が遅（おく）れると、ついイライラしてしまう。
When the train is late, I’ll get angry due to a lapse in judgment.
お腹（なか）が空（す）いていなくても、目（め）の前（まえ）に食（た）べ物（もの）があるとつい食（た）べてしまう。
Even if I am not hungry, if there is food in front of me, I will eat it due to a lapse of forethought.