Kummer used $\lambda$ for a prime (he also used q), see here. I have no clue whether Weil introduced $\ell$ on his own or was following a previous tradition, but $\lambda$ is really close to $\ell$...
I don't know if Kummer introduced $\lambda$ or it was already in use by Jacobi, Dirichlet, or someone else. Does anybody know?

