Merely, just (4)
I believe the answer is:
only
I believe this is a double definition.
'merely' is the first definition.
(synonyms)
'just' is the second definition.
(synonyms)
(Other definitions for only that I've seen before include "Merely; single" , "Siblingless (child)" , "Exclusive - individual" , "Single, solitary" , "See 6" .)