We report on the epitaxial growth of high-quality core-shell nonpolar m-plane GaN/InAlN multiple quantum wells (MQWs) on the sidewall facets of c-oriented hexagonal GaN wires. Pseudomorphic growth without generation of threading dislocations has been established for planar GaN/InAlN (In = 15%) MQWs grown on m-GaN substrates, although m-plane InAlN epilayers cannot be grown perfectly lattice-matched to GaN along the two in-plane directions. Calculations based on elasticity theory indicate that the significant amount of strain oriented along the c-axis is the likely factor favoring the formation of cracks along this direction. For the core-shell wire geometry, such cracks are not observed, leading to high structural quality MQWs. A significant UV emission centered around 3.7 eV at room temperature with a strong polarization perpendicular to the wire axis is observed for those core-shell wires, which is consistent with k.p method calculations, proving the absence of quantum confined Stark effect on nonpolar m-plane surfaces. These excellent optical features reported in the UV spectral range are attributed to the defect-free nature of the GaN/InAlN MQWs, thereby opening promising opportunities for the realization of UV light emitters.