ssäpäätteeseen
Säpäätteeseen is not a standard widely recognized word in Finnish. It does not appear as a defined entry in common dictionaries or grammar references. In many cases, it is likely a typographical error, a dialectal variant, or a nonce form created for illustrating a point about suffixation. Because there is no established meaning, there is no authoritative definition or usage guide for säpäätteeseen in formal Finnish.
One plausible interpretation is that säpäätteeseen is a mistaken or illustrative form related to päätteet, meaning
Because säpäätteeseen is not established in standard usage, any definite analysis depends on context and author
See also: Finnish morphology, illative case, Finnish suffixes (päätteet), word formation, linguistic notation. For authoritative information