This is conjectural, but building on Nick Decroos's answer and cnread's comments, appactim may be intended as an adverb meaning something like "(fixed) fast".
Formally, it's pretty clearly an adverb in -im (as in verbatim etc.), and the stem does seem to be that of pangō "fix, fasten" with the prefix ad-. I'm not finding any attestations for a verb appangō; the Latindict link Nick gives says there's one citation in Lewis and Short, but Perseus seems to know nothing of this, nor does du Cange's massive Latin dictionary. There is a rare verb appingō, though, with the vowel change one would expect in such prefixed verbs, and if this verb is like impingō, its passive participle would be appactus, from which appactim would be regularly formed.
As for what the lexicon entry is trying to say: I'm guessing pr. might stand for propriē "in a proper sense, literally". (Searching for "pr." in the same lexicon yields a number of entries where a more literal meaning marked pr. is followed by a more figurative meaning marked with hinc or inde.) If so, what it's saying is that הֲלוֹם halōm as a particle means "hither", but that its original or literal meaning is appactim.
What does הֲלוֹם actually mean? There are actually two (unrelated I believe, though the lexicon seems to think otherwise) homonyms in Hebrew. הֲלוֹם can be an adverb meaning "hither" (huc, usque huc); but it can also be a form of the verb root h-l-m "pound, strike forcefully" (impegit, contudit). It seems that the lexicon is improbably deriving the "hither" meaning from the "pound" meaning, presumably by a semantic route like "pounded" -> "fixed fast" -> "here" -> "hither".