Derivation of the word "firm".

An interesting detail. Spulber (2008: 5, footnote 8) gives the origin of the word 'firm' as "[t]he word "firm" derives from the Latin word "firmare" referring to a signature that confirmed an agreement by designating the name of the business". According to Cresswell (2021), "[f]irm meaning 'not yielding to pressure' comes from the Latin firmus, also the root of farm [ME], which originally meant a tax or rent. Firm meaning 'a company or business' has the same root, but the immediate origin is different. The Latin word has also given rise to Italian firma 'confirmed by signature', and in the late 16th century this was adopted into English to mean the name under which business was transacted by an organization [M18th], as in 'trading under the firm of "Grant & Co"'. Finally, in the 18th century, firm became the term for a company''. 

Refs.:

• Cresswell, Julia (ed.) (2021). Oxford Dictionary of Word Origins, 3rd edn., Oxford: Oxford University Press.
• Spulber, Daniel F. (2009). The Theory of the Firm: Microeconomics with Endogenous Entrepreneurs, Firms, Markets, and Organizations, Cambridge: Cambridge University Press.