From suwm; a deposit, i.e. Pledging -- + fellowship.
see HEBREW suwm
H8667. tesumeth
תְּשׂ֫וּמֶת noun feminine (deposit, then, construct ׳ת ׳יַד pledge; security, Leviticus 5:21 (P; "" מִּקָּדוֺן).
II. שׂים שׂום, (perhaps √ of following; compare Arabic be inauspicious, ill-luck).