Every noun in Hebrew has a gender and is pre-determined to be either feminine or masculine. The word “ot” is a feminine noun and was first used in the Bible in the plural form: “o-tot.”
“And God said, Let there be lights in the firmament of the heaven to divide the day from the night; and let them be for signs, and for seasons, and for days, and years”
In Genesis 4:15 it is mentioned in the meaning of a “mark”: “And the Lord set a mark upon Cain, lest any finding him should kill him.”
The bow in the cloud was a sign of a covenant (ot-B’rit) between God and mankind after the flood. “Ot” and “otot” were requested as evidence that God the Father was standing behind people and prophets. For example, the Pharisees were asking for signs from Jesus:
“The Pharisees and Sadducees came, and testing him, asked him to show them a sign from heaven”
“Ot” is also a beckon. For example:
“They beckoned to their partners in the other boat, that they should come and help them. They came, and filled both boats, so that they began to sink”
In The Acts 28:11 we see the use of “ot” as a written sign, a kind of a placard: “After three months, we set sail in a ship of Alexandria which had wintered in the island, whose sign was ’The Twin Brothers.’” This is the closest meaning to the main use of this word today: a letter (character).