The affix ṭac comes after the word śvan preceded by ati in a tat-puruṣa samāsa ,
Thus अतिक्रान्त श्वानम् = अतिश्वो वराहः \the boar that surpasses the dog\ i.e. जववान् \very swift\. So अतिश्वो सेवकः \the dog-surpassing servant i. e. more devoted to his master than a dog\. So also अतिश्वी सेवा \service worse than that of a dog\ = अतिनीचासेवा ॥,