A hostile witness.

i). A hostile witness is the one who by his testimony, conducts, attitude or demeanor appears to be biased against the party calling him or is unwilling to tell the truth. 

A hostile witness is the one who tells lies about what he obviously knows, or who deliberately changes his earlier story and from his demeanor and bearing he is clearly biased against the party calling him. 

ii). When a witness has been treated as hostile, the evidence of that witness is hardly worthy of credit. The reason for treating the evidence of a hostile witness as being worth of no credit is that, such witness is unreliable. In Alowo vs. Republic [1972] E.A.324 the Court of Appeal for East Africa said that the basis of leave to treat a witness as hostile is that the conflict between the evidence which the witness is giving and some earlier statements shows him or her to be unreliable and this makes his or her evidence negligible.