Learn Farsi: من ناشیانه بازی کردم و مهره را از دست دادم.

This word comes from the noun or adjective ناشی (meaning an amateur or a clumsy person) combined with the suffix -انه which creates an adverb of manner. Because ناشی ends in a vowel, a 'y' sound is naturally inserted between it and the suffix to make it easier to pronounce, creating ناشیانه meaning clumsily or in an amateur way.

Why is the verb از دست دادم used for losing a game piece instead of گم کردم?

The compound verb از دست دادن literally translates to 'to give from hand'. It is used when you lose possession of something, like losing a game piece to an opponent, an opportunity, or a life. You would use گم کردن when you accidentally misplace a physical object, like losing your keys.

What does the word مهره mean here, and can I use it for a piece of food?

In this context, مهره refers specifically to a playing piece in a board game like chess or backgammon. It can also mean a bead or a mechanical nut. You cannot use it for a piece of food or paper; for those, you would use a word like تکه.

Why is را placed after مهره?

The word را is the definite direct object marker. It shows that the speaker is talking about losing a specific game piece that was just in play, rather than just any piece. This is why the English translation says 'the piece' instead of 'a piece'.