About this word

Deirfiúr is a feminine noun that means sister, in the sense of 'female sibling'.

It is lenited here because of the preceding posessive adjective mo (my).

Notes

This word is derived from the compound dearbh-shiúr (literally, absolute-sister; pragmatically, blood-sister).

The word siúr is used in the sense of sister when there is no blood-relationship; members of female religious orders for example.

Similarly:

• Deartháir, derived from dearbh-bhráthair: blood-brother.
• Bráthair: brother (no blood-relationship; as in members of male religious orders). Also, monkfish!