lig ar

From Middle Irish léicid for (“to feign”). The second sense is calqued from English let on.

lig ar

1. to pretend

   Tá sé ag ligean air féin go bhfuil sé bodhar.

   He's pretending to be deaf.

2. to let on (to reveal or indicate)
   • 1939, Peig Sayers, “Inghean an Cheannaidhe”, in Marie-Louise Sjoestedt, Description d’un parler irlandais de Kerry (Bibliothèque de l'École des Hautes Études; 270) (overall work in French), Paris: Librairie Honoré Champion, page 196:

     Do ghlac rabharta feirge an captaen, ach níor leig sé aon nídh air.

     A flood of rage seized captain was filled with rage, but he didn’t let on.

The preposition ar is reflexive in this construction: its object agrees with the subject of the verb:

• ligim orm ― I let on
• ligeann tú ort ― you let on
• ligeann sé air ― he lets on