A weak high is a high which developed at a visual technical level.  When the market is very visual/technical/mechanical this is a sign that it is controlled by momentum traders. These are medium size market participants like technical hedgefunds which have a relatively short timeframe. The point is, that positions of these shorter timeframe players have to be exited when they go against them. That´s the reason why weak highs can accelate prices when the market trades through them. Everytime a reference is hit and holds it gets weaker.

Weak highs are also bad or unsecure highs.