A Bootlicker is the same as a kiss-ass – a person who will go through all lengths to be favored by people in power or other people considered above them.
They don’t care if the person is destroying the lives of thousands of people or turning a country into ruins. They want to be in their good sides no matter what, hoping that some of their wealth and power will radiate over to themselves.
Everyone recognizes a Bootlicker, also the person they are licking their boots of, and the superior person will often use their kiss-ass tendencies for their own gain, while not giving anything back.
Bootlicker is a figurative speech of someone actually bending down and licking the boots a superior person.
It is inspired by the historical tendency of servants and peasants to lower themselves down to foot level – and sometimes kiss the feet – of kings and other people in power, as to show respect and gain mercy from them.
Some will refer to a Bootlicker in personal terms, for example a friend that is desperately trying to make everyone like them.
In Marxist terms, a Bootlicker is any middle class or lower person who defends a free-market capitalist system, even though they have no gains from it – rather losses.
Trump supporters have also been called Bootlickers by media.