وَمَا كَانَ صَلَاتُهُمْ عِندَ ٱلْبَيْتِ إِلَّا مُكَآءً وَتَصْدِيَةً ۚ فَذُوقُوا۟ ٱلْعَذَابَ بِمَا كُنتُمْ تَكْفُرُونَ
Important Words:
مکاء (whistling) is the noun-infinitive from مکا (with واو as the last root letter). They say مکا الرجل i.e. the man whistled with his mouth; or he brought together his fingers and blew through them, producing a whistling sound (Aqrab).
تصدیة (clapping of hands) is derived from صدی (sadda). They say صدی بیدیه i.e. he clapped with his hands. So تصدیة means, clapping with the hands. الصدی means, the sound or cry returned by a mountain, etc., when one shouts at it; echo (Lane under صدی). Some authorities derive the word تصدیة from the root صد (he turned away), the word الصد signifying the face or front of the hand (Lane under صد).
Commentary:
The verse describes the sacrilegious use of the Sacred Mosque by idolaters. Instead of using it for worship and devotion, they used it for talking and gossiping and for whistling and the clapping of hands.
The words, And their prayer at the House is nothing but whistling, etc. mean: (1) that even their acts of worship consisted of nothing but whistling, etc.; or (2) instead of praying and performing acts of devotion, they passed their time in whistling, etc.