רעל

Where it comes from

The word רַעַל appears once in the Bible in Zechariah 12:2 and understood there differently by different commentators and translators. Some understand it as "reeling" or "staggering", which corresponds to the Aramaic meaning of the identically-written word, so סף־רעל means "a cup that causes staggering", because it is intoxicating like wine. Others, like Rashi and David Altschuler (Metsudat David), interpreted it as "a killing drug", i.e. poison. The latter became the word's primary meaning in Modern Hebrew.

The modern military slang sense of "morale" is influenced by the expression רוח הלחימה, "fighting spirit", which is abbreviated רה״ל, which, in turn, is pronounced similarly to רעל (see References). The root is perceived to be the same as the root of "poison", so for example, in this context, to "poison" someone (הרעיל) means to boost their morale.