Those who do not know about Lakshman rekha – rekha means line which was drawn by lakshmana for the protection of sita, when he went in search of Raama.
But later on this word lost its original significance and was used to put a line in order to keep the cockroaches away. In fact I have heard people asking in the shops for lakshman rekha, instead of cockroach hit. The other day when I was cleaning the house I saw cockroaches moving around my maid immediately told me to put lakshman rekha. If Lakshmana were to hear this may be he would also see the funny side of it and would justify by comparing the cockroach with raavana.