दृढः स्थूलबलयोः

The irregularly formed Past Participle दृढ means 'stout' and 'strong'.
It is derived from दृंह् with क्त affix. In other senses, the forms are दृंहितम् or दृहितम् ॥ There are two roots one दृह् (1.769) without nasal, and the other दृंह् (1.770) with the nasal. दृढ can be derived from any one of these by eliding ह, and the nasal, and changing त to ढ, and not adding the augment इट् ॥

The difference between स्थूल and बल is that a man may be stout or स्थूल without being strong (बलवान्) and vice versa. The word बल in the sutra is equal to बलवत्; in fact, the word बल is formed by अच् affix. The irregularity in the formation of दृढ consists in the absence of इट् and the elision of ह् (and of न्, if the root दृंह be taken) : and the change of त into ढ ॥ This irregular elision of ह is for the sake of preventing the application of the rule पूर्वत्रासिद्धम् 8.2.1. The form could have been obtained in the regular way by the elision of ढ, thus : दृह् + त = दृढ् + त 8.2.31 = दृढ् + ध = दृढ् + ढ = दृ + ढ 8.3.13 = दृढ ॥ But then when ढ is elided, the rule पूर्वत्रासिद्धम् will apply, and the forms द्रढिमा, द्रढीयान्, द्रढयति could not be obtained. For ॠ is changed to र by 6.4.161 only when it is laghu or light, but ढ -lopa being considered asiddha, the ॠ would be heavy as standing before a conjunct consonant. So also the form परिदृढय्याऽगतः could not be obtained: for the णि would not be changed to अय् before ल्यप् when the ॠ is not light or laghu 6.4.56. So also, परिदृढस्यापत्यं = परिदृढी (the daughter of Paridrdha) could not be formed. For ॠ being considered guru or heavy, the affix ष्यङ् would have come in the feminine 4.1.78.