Solid, as muscles or a mattress

adjective

1. Securely or solidly fixed in place

   "his teeth were firm"

2. Not weak or uncertain vigorous

   "a firm handshake"

3. Having a solid or compact structure that resists stress or pressure

   "firm apples"

4. Not easily moved or disturbed steadfast

   "a firm believer in democracy"

5. Well-founded

   "firm evidence of criminal activity"

6. Indicating firmness or resolution

   "a firm mouth"

adverb

1. In a firm manner steadfastly, fixedly

verb

1. To make secure or fast tighten

   "firming her grip on the racquet"

2. To make solid or compact

   "firm the soil"

3. To put into final form settle

   "firm a contract"

4. To give additional support to strengthen
5. To become firm harden
6. To recover from a decline improve

   "the market is firming"

noun

1. The name or title under which a company transacts business
2. A partnership of two or more persons that is not recognized as a legal person distinct from the members composing it
3. A business unit or enterprise

idiom

1. To refuse to change what one has been doing or believing

   "They are holding firm on their refusal to proceed."

noun

1. A group of lawyers who work together as a business

idiom

1. Well supported

   "He believes that his argument is on firm ground/footing."

idiom

1. To refuse to change a decision, position, etc.

   "The judge stood firm in her ruling."

idiom

1. To treat someone in a firm way

   "He took a firm line with his son."

adjective

1. Occurring between or involving two or more firms

   "interfirm transactions"