  Using Monte Carlo simulations with a tunable uniaxial strain, for the first
time the nematic susceptibility of a model for the pnictides is calculated. The
results are in good agreement with the experiments by J-H. Chu {\it et al.},
Science {\bf 337}, 710 (2012). Via a Ginzburg-Landau analysis, our study
suggests a nematicity in pnictides primarily originating on magnetism, but with
the lattice/orbital boosting up critical temperatures and separating the
structural $T_S$ and N\'eel $T_N$ transitions. At $T>T_S$, Curie-Weiss behavior
is observed with the characteristic temperature $T^*$ unveiled by Chu {\it et
al.} being the $T_N$ of the purely electronic system. In this temperature
regime, short-range magnetic order with wavevectors $(\pi,0)-(0,\pi)$ induce
local nematic fluctuations and a density-of-states pseudogap, compatible with
several experiments.
