  We have investigated the structural and magnetic transitions in CsCoO$_2$
using calorimetric measurements, neutron powder diffraction (NPD), density
functional theory (DFT) calculations and muon-spin relaxation ($\mu$SR)
measurements. CsCoO$_2$ exhibits three-dimensional long-range antiferromagnetic
(AFM) order at 424~K, resulting in antiferromagnetic alignment of chains of
ferromagnetically ordered Co-Co spin dimers. Although there is no change in
magnetic structure around a structural transition at $T^{*}=100$~K, the
resulting bifurcation of corner-shared Co--O--Co bond angles causes a weakening
of the AFM interaction for one set of bonds along the chains. Consequently, the
system undergoes a complex freezing out of relaxation processes on cooling.
