sheva

The Hebrew number for seven is שבע - sheva. A homonymic root is שבע - "to swear, to take an oath". Is there a connection between the two? There seems to be. Klein says the verb derives from sheva (seven) and "prob. meaning lit.: 'to bind oneself by seven things, or by seven oaths'." Steinberg writes that the number seven was holy to the ancient peoples, based on their view that there were seven planets (they included Sun, Moon, Mercury, Venus, Mars, Jupiter, and Saturn). The connection between "seven" and "oath" becomes clear in the stories relating to the founding of Beer Sheva, both with Avraham and with Yitzchak. The name appears both as meaning "Well of Seven" and "Well of Oath".

Of course a major derivative of sheva is shavua שבוע - meaning week. R' Hirsch goes so far as to say that the connection mentioned above between seven and oath is also related to shavua: "An oath thus obligates a person through everything that was made in the seven days of creation " (Bereshit 21:31).

One last note: an occasional error (found here for example) is to think that the mourning period known as shiva derives from the Hebrew root for sitting - ישיבה. The actual root is the masculine form of the number - שבעה - shiva, for the "seven days of mourning."