Mid 19th century; earliest use found in Fraser's Magazine for Town and Country. Partly from prothetic + -ally, and partly from prothetical + -ly; compare -ically.
Definition of prothetically in US English:
prothetically
adverb
rare
1In a prothetical manner.
2Grammar. By prothesis or prefixion.
Origin
Mid 19th century; earliest use found in Fraser's Magazine for Town and Country. Partly from prothetic + -ally, and partly from prothetical + -ly; compare -ically.