Grin means to smile happily
First
Prime does mean first.
If you mean "prime" rather than "prim", then no. The nearest primes are 139 and 149 if that helps.
Great big grin
The future tense of "grin" is "will grin."
plants
The term "mean long grin" is not commonly used or recognized. It is likely a made-up phrase or a specific term used in a certain context or community. Without further information, it is difficult to provide a specific meaning for this phrase.
Lombardies are trees, but in this context, prim means they are well maintained. They are trimmed so they stand tall and straight, not wild and bent.
No, "grins" is not the plural of "grin." "Grins" is the present tense form of the verb "grin." The plural of "grin" is "grins."
In texting, g or <g> means "grin".
=D is a cheesy grin :)