  We present \texttt{ethraid}, an open source Python package designed to
measure the mass ($m_c$) and separation ($a$) of a bound companion from
measurements covering a fraction of the orbital period. \texttt{ethraid}
constrains $m_c$ and $a$ by jointly modeling radial velocity (RV), astrometric,
and/or direct imaging data in a Bayesian framework. Partial orbit data sets,
especially those with highly limited phase coverage, are well-represented by a
few method-specific summary statistics. By modeling these statistics rather
than the original data, \texttt{ethraid} optimizes computational efficiency
with minimal reduction in accuracy. \texttt{ethraid} uses importance sampling
to efficiently explore the often broad posteriors that arise from partial
orbits. The core computations of \texttt{ethraid} are implemented in Cython for
speed. We validate \texttt{ethraid}'s performance by using it to constrain the
masses and separations of the planetary companions to HD 117207 and TOI-1694.
We designed \texttt{ethraid} to be both fast and simple, and to give broad,
"quick look" constraints on companion parameters using minimal data.
\texttt{ethraid} is pip installable and available on Github.
