  We revisit bulk FeSe through the systematic first-principles electronic
structure calculations. We find that there are a series of staggered $n$-mer
antiferromagnetic (AFM) states with corresponding energies below that of the
collinear AFM state which is the ground state for the parent compounds of most
iron-based superconductors. Here the staggered $n$-mer ($n$ any integer $>1$)
means that a set of $n$ adjacent spins parallel on a line along $b$-axis with
spins in antiparallel between $n$-mers and along $a$-axis. Among them, the
lowest energy states are quasi-degenerate staggered dimer and staggered trimer
AFM states as well as their any staggered combinations. Thus, to have the
largest entropy to minimize the free energy at low temperature, the most
favorable state is such a quasi-one-dimensional antiferromagnet in which along
$b$-axis a variety of $n$-mers, mostly dimers and trimers, are randomly
antiparallel aligned while along $a$-axis spins are antiparallel aligned, i.e.
actually a nematic paramagnet. This finding accounts well for the absence of
long-range magnetic order in bulk FeSe and meanwhile indicates the dominant
stripe spin fluctuation and the nematicity as spin-driven.
