The English word “kosher” means “fit” or “proper”. “Kosher” as an English word - which is an official part of the English language and found in any dictionary - is actually derived from the Hebrew word “kasher”, which is found in the Scroll of Esther and the Book of Ecclesiastes.
Queen Esther said to King Achashverosh, “If it please the King, and if I have found favor before him, and the idea is proper to the King, and I am pleasing in his eyes, let an order be issued ordering the withdrawal of the letters containing the plot of Haman, son of Hamdata, the Agagite, in which he ordered the destruction of the Jews throughout the King's provinces.” (Esther 8:5)
The Hebrew word in this verse for “is proper” is the word “kasher” – “kosher.
In Ecclesiastes we actually find the root-word “kasher” in two different forms in two verses – 2:21 and 11:6 - but the second one highlights the meaning of the word in the way we use it today. The verse states, “In the morning plant your seed, and do so also in the evening, because you do not know which will be suitable, this or that, or whether they both shall be good.” The Hebrew word in this verse for “will be suitable” is “yichshar”. This means “will be kosher” in the sense that it will be fit and good.
Although mostly thought of as a word that is exclusive to food items, it may be used to describe any object or person that is proper and fit. Such as “The deal seems kosher” or “David is a kosher person for the job”. In fact, in Talmudic times (c. 500 CE) “kosher” was mainly used to describe the fitness of an item for a mitzvah, unlike today where the primary use is for “kosher foods”.
The Torah does not speak about food in terms of being “kosher” or not. Rather the verses state “Eat this type of food…” or “Don’t eat this type of food…”. However, today we use the word “kosher” on a daily basis regarding food, “This food is kosher”, “This food is not kosher”. I should also point out that there are degrees of being “kosher”. “This food is very kosher (usually called “glatt kosher”).
I hope that you find this answer helpful, fit and “kosher”!