1) with the whole multitude
2) all together, one and all
Part of Speech: adverb
Dative case (adverb) of a compound of G3956 and G4128; in full multitude, that is, concertedly or simultaneously: - all at once.
These two sites give similar information, with the definition from several dictionaries and statistics on the use of the word.
Perseus Digital Library
University of Chicago's Logion lexicon
Thayer's dictionary plus other information.
From this site's dictionary (in Italian)
Click on the first column to search for that word as a form of the root παμπληθεί; click on the second column to search for that grammatical form of the root παμπληθεί; click on the third column to search for that word and grammatical form; click on the total to search for the root παμπληθεί.