  We studied in detail the propagation of a massive tachyonic scalar field in
the background of a five-dimensional ($5D$)
Einstein--Yang--Mills--Born--Infeld--dilaton black string: the massive
Klein--Gordon equation was solved, exactly. Next we obtained complete
analytical expressions for the greybody factor, absorption cross-section, and
decay-rate for the tachyonic scalar field in the geometry under consideration.
The behaviors of the obtained results are graphically represented for different
values of the theory's free parameters. We also discuss why tachyons should be
used instead of ordinary particles for the analytical derivation of the
greybody factor of the dilatonic $5D$ black string.
