This word has the root, רגל, regel, which means "foot", also related to "ragal" which means to "go about on foot". The connotation is for those who go about on foot in order to seek out slander, or for malicious intent. One place which highlights this meaning is in B'reisheet (Genesis) 42:31, when Joseph's brothers are defending themselves and say:
כֵּנִים אֲנָחְנוּ: לֹא הָיִינוּ, מְרַגְּלִים
"honest we are, we are spies!"
(I know, Hebrew syntax is very Yoda-like).
Have fun with Hebrew! I would love if you would send me a word to translate or a passage to analyse.