"In His Service" is a commonly understood meaning of the abbreviation.
However, according to the Catholic Encyclopedia (the first major source on
Google that appeared),  A monogram of the name of Jesus Christ. From the
third century the names of our Saviour are sometimes shortened, particularly
in Christian inscriptions (IH  and XP, for Jesus and Christus). In the next
century the "sigla" (chi-rho) occurs  not only as an abbreviation but also as
a symbol. From the beginning, however, in Christian inscriptions the nomina
sacra, or names of Jesus Christ, were  shortened by contraction, thus IC and
XC or IHS and XPS for Iesous Christos.
                          These Greek monograms continued to be used in Latin
during the Middle Ages.   Eventually the right meaning was lost, and
erroneous interpretation of IHS led to   the faulty orthography "Jhesus". In
Latin the learned abbreviation IHC rarely occurs after the Carlovingian era.
The mongram became more popular after the twelfth century when St. Bernard
insisted much on devotion to the Holy Name of Jesus, and the fourteenth, when
the founder of the Jesuati, Blessed John Colombini (d. 1367), usually wore it
on his breast. Towards the close of the Middle Ages IHS became a symbol,
quite like the chi-rho in the Constantinian period.

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