jessu għalik

Etymology
. rejects a derivation from. The required contraction of into  is indeed unlikely. However, Aquilina’s own connection with seems even less plausible.

The phrase is reminiscent of dialectal, which often has the same sense of “bravo!, well done!”. The word would then be Christian. The unexpected gemination is not problematic in this exclamatory context. However, it is questionable whether was ever used in Malta. While there were Arabic-speaking missionaries, no Christian Arabic names are found. Moreover, the Islamic name form is attested (though it is not entirely clear whether the Maltese actually used it among themselves or knew it mainly through Muslim slaves).

Interjection

 * 1)  bravo!, well done!