  By means of a combined x-ray diffraction, magnetic susceptibility and
specific heat study, we investigate the interplay between orthorhombic
distortion and stripe-like antiferromagnetic (AFM) order in the Mott insulator
BaCoS$_{2}$ at $T_N=290$ K. The data give evidence of a purely electronic AFM
transition with no participation of the lattice. The observation of large
thermal fluctuations in the vicinity of $T_N$ and a Schottky anomaly unveils
competing ground states within a minute $\sim$1 meV energy range that differ in
the orbital and spin configurations of the Co ions. This interpretation
suggests that the stripe-like order results from a spontaneous symmetry
breaking of the geometrically frustrated pristine tetragonal phase, which
offers an ideal playground to study the driving force of multi-orbital Mott
transitions without the participation of the lattice.
