Tough love is an expression for which the first account I could track down was the title of a book called "Tough Love" by Bill Milliken, published in 1968.
There are several ways that tough love is used in everyday language. Most commonly, it is used to describe any type of parenting in which the child experiences some negative emotions as part of a learning process. This can range from healthy setting of firm boundaries, common in authoritative parenting styles, to abusive parenting styles in which humiliation, belittling or physical violence are used to control the child.
Tough love can, therefore, refer to a positive approach to parenting in which the child learns valuable lessons, but does not suffer the ill effects of abuse, because it is not abusive, and preserves the dignity of the child. It could equally be applied to a harsh approach to parenting in which the child's self-esteem is undermined, and they are subject to physical, emotional or even sexual abuse. In that respect, tough love becomes virtually meaningless other than to denote some discomfort on the part of the child, and cannot be used to infer either the intention or the legitimacy of the parent's style of discipline.
For this reason, the concept of tough love has been widely criticized as an excuse used to bully children, in particular, in the context of bullying teens who are using drugs. Some youth drug treatment programs use the term tough love to describe their approach, and which may be interpreted to mean that the approach is harsh, and may be effective only by breaking down the will of the young person in treatment.