shalosh

The feminine form of three in Hebrew is שלוש shalosh, and the masculine form is שלושה shlosha. All the numbers from three to ten follow this pattern, where the masculine ends in ה ָ (ah) and the feminine does not. This is the exception to the general trend - with most nouns, verbs and adjectives the feminine ends in ה ָ (ah) and not the masculine. What's the story here?

As of now, the only explanation I could find is in Steinberg's Milon HaTanach (in the entry for ארבע). He says he discusses it more in מערכי לשון עבר - Maarchei Lashon Avar 127, 28 - which was published in 1884. He claims that originally the suffix ה ָ (ah) was used for both masculine and feminine numbers. He gives examples from Melachim I 7:30, Yechezkel 7:2 and Yirmiyahu 36:23. In time when they began differentiating between genders for numbers, they dropped the last syllable in the feminine form. He says a similar development occurred with the male word for "you" - אתה atah, and the feminine את at.

A derivative of shalosh is shalish שליש. This word has three meanings:

a) A dry measure of volume (found in Tehilim 80:6, and Yishayahu 40:12). Klein states it is likely a third of an ephah.

b) A musical instrument, as mentioned in Shmuel I, 18:6. This was either three-sided (like the triangle in English) or three-stringed.

c) An officer in the army, first used in the Torah to describe Egyptian officers, and still used today in the IDF. There are those who claim that the word is not connected to the number three (Onkelos, Rashi on Shmot 14:7) and the Daat Mikra mentions that shalishim in Ugaritic refers to soldiers who were trained to shoot arrows from a moving chariot. However, there are two other theories that do connect shalish to shalosh. One claims that the shalish was third in command, and another theory states that there were three soldiers in every chariot.

Another derivative of shalosh is shilshom שלשום - the day before yesterday. Klein says it literally means "three days ago".