Could you please tell me the meaning of "tense" in this excerpt from the first chapter of The Great Gatsby? Does it mean "firm", "solid"?
"Before I could reply that he was my neighbor dinner was announced; wedging his tense arm imperatively under mine Tom Buchanan compelled me from the room as though he were moving a checker to another square."
"Tense" here connotes Tom's mood and character as well as describing, as you correctly infer, the firmness and strength of Tom's arm as he guides Nick out of the room.
"Tense" is here used in its definitive meaning as an activation of musculature as well as carrying associations with Tom's state of mind. "Tense" and "imperative" combine to demonstrate the overbearing, self-assured and brutish nature of Tom's character.
We see Fitzgerald here establishing Tom as a person who uses his wealth and physical strength to dominate others.
The overall impression the reader has of this character is his physical power and brute strength.
Also notable here is the contrast between Tom and the descriptions of Daisy and Jordan, who are associated with softness and ethereal qualities in the opening chapter.