In Act 4, Scene 1 of Romeo and Juliet, Paris is with Friar Lawrence. He speaking to the Friar of his upcoming marriage to Juliet when she herself walks in on them. After a few minutes of dialogue between Juliet and Paris wherein Paris is discussing Juliet's confession (which he assumes is the reason Juliet has come to see the Friar), the Friar asks him to leave. When Paris leaves he says, "Till then, adieu, and keep this holy kiss" as he kisses Juliet. The term "holy kiss" is in reference to their soon-to-be marriage; they are to be married in the church, a holy sacrament, and Paris considers them already to be married in the sense that Juliet's father has promised her to him. He believes also that Juliet is still chaste; a requirement for holy marriage and a misbelief on his part.
Of course, this upsets Juliet on one level because she is already married, and her husband has been banished. She has gone to the Friar to ask for his help, and if no help is to be found, plans to commit suicide.
The term "holy kiss" is also significant as a parallel to the dialogue between Romeo and Juliet when they first meet in Act 1, Scene 5. Romeo, instantly in love with Juliet, brings up the idea of kissing. He calls his hand "unworthiest" and calls his lips pilgrims ready to kiss. Juliet, rather more bashful, says to him:
Good pilgrim, you do wrong your hand too much,
Which mannerly devotion shows in this,
For saints have hands that pilgrims' hands do touch,
And palm to palm is holy palmers' kiss.
When their hands touch, it could be considered a symbolic first kiss (although they soon exchange a real kiss). This touching of palms is the first reference in the play to a "holy kiss." Presumably, Juliet had never been kissed before this night. This scene is the first intimate moment that Juliet and Romeo share, and it would naturally upset Juliet on a second level to hear the term "holy kiss" from Paris as he kisses her, as it disgraces that first, cherished encounter.