The words puṣya and siddhy are irregularly formed by the affix kyap , when used as names of asterisms.,
Thus पुष् + क्यप् = पुष्यः 'the Pushya asterism'. It is so called because objects are nourished under the influences of this asterism. सिध् + क्यप् = सिद्ध्यः 'the asterism Siddhya' another name of Pushya, so-called because things are accomplished under the influence of this star.
When not the names of asterisms, the forms are पोषणं 'nourishing', सेधनम् 'accomplishing'.,