From Wiktionary, the free dictionary
pèin
- second-person plural of fhèin
- Used in conjunction with the pronoun sibh and its prepositional forms, e.g. sibh pèin, agaibh pèin, etc.
- Sometimes the combination is written as sib' fhèin, agaib' fhèin, etc., instead, and sometimes the pronunciation is not indicated in writing at all.