epenthesis, or more specifically, excrescence
In phonology, epenthesis (/əˈpɛnθəsɪs/; Ancient Greek: ἐπένθεσις) means the addition of one or more sounds to a word, especially to the interior of a word. … excrescence, for the addition of a consonant, …
"The history of English provides examples [of epenthesis] like the development of aemtig into empty, with epenthetic p, and of þunor into thunder, with epenthetic d. Non-standard pronunciations include 'athalete' for athlete and 'fillum' for film,' with epenthetic vowels."
(R.L. Trask, A Dictionary of Phonetics and Phonology. Routledge, 1996)
Consonant epenthesis (…) arises in order to provide a more consonantal syllable coda. There are some words in English originally which ended in an alveolar nasal or an /s/ and which developed an epenthetic stop after the final segment. The result is that the syllable rhyme of such words shows a steady decrease in sonority from the nucleus to the right edge.
vowel – nasal: sound (< son)
fricative – stop: against (< againes)
It's helpful also to read on the related idea of Sandhi (Sanskrit: संधि saṃdhí 1 "joining") for the morphology.