ām is the affix of the verb kās 'to cough' and of those roots that are formed by affixes i.e. the derivative verbs, when liṭ (Perfect Tense) follows, in the mantra,
This forms the Periphrastic Perfect. The Perfect of कास् will be कासाम्, after which the terminations of the Perfect are all elided 2.4.81 and instead, the perfect tense of the auxiliary verb भू or अस् is added. As कासाञ्चक्रे 'he coughed'; चोरयामास 'he stole'. So also लोलूवाञ्चक्रे.
This rule however does not apply to the Vedas; as कृष्णो नोनाव Rig. Veda (I. 79.2).
Vart:- This rule also applies where the root is polysyllabic consisting of more than one vowel; as चुलुम्पाञ्चकार, दरिद्राञ्चकार ॥
The final म् of आम् is not indicatory.,