ekki með fulle fem

"Fulle fem" is Danish/Norwegian for "complete five", so "having one's complete five" likely refers to being in possession of all of one's five senses (thus being mentally fit). This phrase is used in modern Norwegian: ved sine fulle fem.

(Can this⁽⁺⁾ etymology be sourced?)

ekki með fulle fem

1. (idiomatic) to be lacking in mental faculties (temporarily or permanently), to be not quite all there, to be delusional

• The positive form "með fulle fem" (to be mentally fit) is also used but not as often.
• "Fulle fem" is not an Icelandic term, here it is viewed more as a mildly light-hearted quotation of Danish/Norwegian.

• vera ekki með öllum mjalla
• ruglaður
• vera ekki með réttu ráði