פירוש על ברכות 110:19

Spontaneously. Or it may refer to hearing a chance verse quoted, e.g. by school-children, this being regarded as a kind of oracle; cf. T. A. III. p. 228, and for some examples from Rabbinic literature, ibid. p. 353 n. 229. It may be compared with the practice of opening the Bible in haphazard manner to find in the page a helpful reference.