The masturbatory act of propping oneself against a bedpost, couch, or bedside and rubbing the clothed penis against it repeatedly, in order to illicit an initial pleasure and an eventual intense, (or otherwise) ejaculation into the seams of the underpants. This, being usually an early form of conventional masturbation, does not at first neccesarily illicit ejeculation. Rather intense feelings of pleasure. Once sexual maturity is reached, and in the rare case the masturbator has not moved on to conventional masturbation, the ejaculation becomes a natural part of the bed-posting process. Most bed-posters move on to conventional masturbation before age 13, but in rare cases, continue well into their teens.
That girl was so hot, I had to go home and start bed-posting to her image.