  We present the results of the monitoring campaign of the Type IIn supernova
(SN) 2018cnf (aka ASASSN-18mr). It was discovered about 10 days before the
maximum light (on MJD = 58293.4+-5.7 in the V band, with MV = -18.13+-0.15
mag). The multiband light curves show an immediate post-peak decline with some
minor luminosity fluctuations, followed by a flattening starting about 40 days
after maximum. The early spectra are relatively blue and show narrow Balmer
lines with P Cygni profiles. Additionally, Fe II, O I, He I and Ca II are
detected. The spectra show little evolution with time, with intermediate-width
features becoming progressively more prominent, indicating stronger interaction
of the SN ejecta with the circumstellar medium. The inspection of archival
images from the Panoramic Survey Telescope and Rapid Response System
(Pan-STARRS) survey has revealed a variable source at the SN position, with a
brightest detection in December 2015 at Mr = -14.66+-0.17 mag. This was likely
an eruptive phase from the massive progenitor star started from at least
mid-2011, and that produced the circumstellar environment within which the star
exploded as a Type IIn SN. The overall properties of SN 2018cnf closely
resemble those of transients such as SN 2009ip. This similarity favours a
massive hypergiant, perhaps a luminous blue variable, as progenitor for SN
2018cnf.
